Field of the Invention
The present invention relates to a lithography apparatus and an article manufacturing method.
Description of the Related Art
An exposure apparatus is an apparatus that exposes a substrate (e.g., a wafer, a glass plate, or the like where a resist layer is formed on the surface thereof) via a pattern of an original (reticle or the like) in a lithography step included in manufacturing steps for a semiconductor device, a liquid crystal display device, and the like. In the exposure apparatus, a very high accuracy is required for focusing and leveling adjustment by a substrate stage with increasing the NA (numerical aperture) of a projection optical system for forming the image of the pattern on a substrate. For example, in the scanning exposure apparatus having an NA of 0.93, the depth of focus which can be ensured during exposure is about 200 [nm]. Here, the accuracy of control which can be allocated to focusing and leveling adjustment is considered. At this time, when error factors such as a field curvature, a defocus amount caused by a reticle, calibration (measurement) of an image plane, and the like are removed from the depth of focus, the accuracy of control is typically about 10% of 200 [nm](about 20 [nm]). Thus, a measuring device that is capable of measuring the distance between the top surface (top plate surface) of a stage and a surface plate on which a projection optical system is secured and includes a laser interferometer exhibiting excellent linearity and high resolution has conventionally been employed for focusing (the Z-axis) and leveling (ωx and ωy) measurement.
On the other hand, Japanese Patent Laid-Open No. H10-163099 discloses an exposure apparatus that includes a plurality of substrate stages in one apparatus for the purpose of making an improvement in throughput. In the exposure apparatus, for example, two regions, i.e., the first region (exposure station) for performing exposure and the second region (measuring station) for performing measurement (alignment measurement) of a substrate prior to exposure are provided on a surface plate. In accordance with this arrangement, the exposure apparatus can expose the first substrate held by the other substrate stage on the exposure station while the exposure apparatus measures the second substrate held by one substrate stage on the measuring station.
The exposure apparatus disclosed in Japanese Patent Laid-Open No. H10-163099 includes a ZX bar mirror as one of flat mirrors (reference members) for use in measurement by a laser interferometer. The ZX bar mirror is disposed on each of the surface plates (surface plate for supporting a projection optical system or a measuring device) of the exposure station and the measuring station so as to face the substrate stage. Thus, two different ZX bar mirrors are referenced by the exposure station and the measuring station, and a focus error may occur on the exposure station if a relative error between the shapes of two ZX bar mirrors exists. Leveling (in particular, ωy) is also based on measurement values obtained by two different ZX bar mirrors, and thus, a leveling error may occur as in the focus error.
Such a relative error between the shapes of two ZX bar mirrors is classified into an error, which can be corrected prior to operation of the apparatus, caused by processing, assembling, installation, and the like and an error over time which appears after operation of the apparatus. Among them, the former error can be corrected by measuring an error between the shapes of two ZX bar mirrors disposed on two stations and storing a correction value corresponding thereto in a memory. On the other hand, in the latter error, firstly, the X-component of an error between shapes having an order less than two order on the Z-X plane in which the entire ZX bar mirror is shifted in the Z-axis direction or the entire ZX bar mirror is tilted may be corrected by zero-point matching between two stations. Here, the error component between the shapes having an order less than two order refers to the X-component of zero order and first order when Z=f(X). However, for example, the error component between the shapes having an order greater than two order, such as an arcuate error component between shapes, cannot be corrected by zero-point matching. Thus, a change over time of the relative error component between the shapes having an order greater than two order between two ZX bar mirrors provides a reduction in positioning precision, which may result in defocus or the like including blur on one side of a projected image (blur caused by a relative tilt between a substrate surface and an image plane).